1. Field of the Invention
The present invention relates to an adjustment method of characteristics of a multistage Mach-Zehnder interferometer type optical circuit, and to a multistage Mach-Zehnder interferometer type optical circuit whose characteristics are adjusted by the same method.
2. Description of the Related Art
As a means for transmitting large amounts of information, a WDM (Wavelength Division Multiplexing) communication system in an optical region is widely used. In the WDM communication system, a multistage Mach-Zehnder interferometer type optical circuit including symmetrical Mach-Zehnder interferometers or asymmetrical Mach-Zehnder interferometers, or both of them, connected in cascade is used as a device capable of implementing a characteristic variable wavelength filter, dispersion compensator, gain equalizer and the like.
To achieve desired characteristics in the multistage Mach-Zehnder interferometer type optical circuit, it is necessary to set the optical path length difference between individual interferometers accurately on an order less than the wavelength of the optical signal. However, the optical path length difference can deviate from a design value because of errors in the fabrication process, thereby often causing a phase error. As a means for compensating for the phase error, phase controllers for controlling the refractive indices of the individual interferometers are provided so that they carry out the phase control of the optical signal in the interferometers by controlling the refractive indices. Accordingly, it is necessary to measure phase characteristics of the individual interferometers before carrying out the control by the phase controllers. An adjustment method of the characteristics of the multistage Mach-Zehnder interferometer type optical circuit will be described with reference to FIGS. 1 and 2.
FIG. 1 is a diagram illustrating a conventional adjustment method of the characteristics of the multistage Mach-Zehnder interferometer type optical circuit, and FIG. 2 is a graph illustrating an intensity characteristic of the optical output when changing phase φ by using the phase controllers in symmetrical Mach-Zehnder interferometers.
FIG. 1 is a schematic diagram of a typical configuration of the multistage Mach-Zehnder interferometer type optical circuit including symmetrical Mach-Zehnder interferometers and asymmetrical Mach-Zehnder interferometers alternately connected in cascade.
More specifically, each symmetrical Mach-Zehnder interferometer comprises a directional coupler 211 placed forward for splitting/combining the optical signal; a directional coupler 212 placed backward for splitting/combining the optical signal; two optical waveguides placed between the directional couplers 211 and 212 and adjusted such that they have the same optical path lengths; and a phase controller 221 attached to one of the two optical waveguides between the directional couplers 211 and 212 to control the phase of the optical signal. On the other hand, each asymmetrical Mach-Zehnder interferometer comprises a directional coupler 212 placed forward for splitting/combining the optical signal; a directional coupler 213 placed backward for splitting/combining the optical signal; two optical waveguides placed between the directional couplers 212 and 213 and adjusted such that they have different optical path lengths; and a phase controller 222 attached to one of the two optical waveguides between the directional couplers 212 and 213 to control the phase of the optical signal. The multistage Mach-Zehnder interferometer type optical circuit includes the symmetrical Mach-Zehnder interferometers and asymmetrical Mach-Zehnder interferometers with the foregoing structures connected in cascade alternately with sharing the directional coupler 212. In addition, at the initial end of the multistage Mach-Zehnder interferometer type optical circuit, there are provided optical waveguides 241 and 242 as an input section of the optical signal.
Since the multistage Mach-Zehnder interferometer type optical circuit has a multi-stage configuration that connects the foregoing symmetrical Mach-Zehnder interferometers and asymmetrical Mach-Zehnder interferometers alternately in cascade, FIG. 1 designates the directional couplers by reference numerals 211-21M, and the phase controllers by 221-22L. In addition, it designates monitoring ports by reference numerals 231-234, an interferometer to be measured by 25, and interferometers section including the monitoring ports by 26. Although the directional couplers 211-21M are illustrated by a single line, they actually include two optical waveguides placed in parallel with appropriate proximity.
The conventional method of adjusting the characteristics of the multistage Mach-Zehnder interferometer type optical circuit with the foregoing configuration is carried out as follows. It provides the circuit with monitoring ports in close proximity at forward and backward stages of the interferometer to be measured just as the interferometers section 26 equipped with the monitoring ports 231-234 as shown in FIG. 1 (K. Takiguchi et al., IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 6, NO. 1, pp. 86-88 (1994)). For example, to adjust the characteristics of the interferometer 25 to be measured, the optical signal is input from the monitoring port 231 or 232 installed at a forward stage of the interferometer 25 to be measured. Then, while changing the phase of the optical waveguide with the phase controller 22J−1 of the interferometer 25 to be measured, intensity changes of the output light from the monitoring port 233 or 234, which is installed at a backward stage of the interferometer 25 to be measured, are measured. Incidentally, the individual monitoring ports 231-234 are connected to the forward and backward stages of the interferometer 25 to be measured by the directional couplers 21K−5, 21K−4, 21K+1, and 21K+2, respectively. In addition, directional couplers 21K−2 and 21K−1 in the interferometer 25 to be measured are provided for connecting other monitoring ports for other interferometers to be measured at the forward or backward stage.
Across the monitoring ports 231 and 233 provided for the interferometer 25 to be measured, a symmetrical Mach-Zehnder interferometer is configured with two optical waveguides having the same optical path lengths. The intensity in the output light changes in response to the phase change φ by the phase controller 22J−1 as illustrated in FIG. 2. Accordingly, measuring the intensity changes makes it possible to know the driving amount (corresponding to a phase control signal) of the phase controller 22J−1 of the interferometer 25 to be measured, and to provide the phase of the optical signal with desired characteristics by setting an appropriate driving amount to the phase controller 22J−1. The graph of FIG. 2, illustrating the phase-output light intensity normalizes the intensity by the maximum intensity under the assumption that the coupling ratio of the directional couplers 21K−3 and 21K is 50% (3 dB directional coupler).
FIG. 1 shows a configuration in which only the interferometers section 26 has the monitoring ports 231-234 to make the difference clear between the section with the monitoring ports and sections without using them. In actuality, however, monitoring ports are provided to the forward and backward stages of all the interferometers except for the interferometers at both ends constituting input and output sections of the multistage Mach-Zehnder interferometer type optical circuit, so that the other interferometers are also subjected to the measurement of the phase characteristics and the adjustment of the phases using the same procedure. The interferometers at both ends of the multistage Mach-Zehnder interferometer type optical circuit constituting its input and output sections can use the optical waveguides constituting the input ports or output ports as the monitoring ports, thereby obviating the need to install the monitoring ports.
The conventional characteristic adjustment method, however, has the following problems because the multistage Mach-Zehnder interferometer type optical circuit must include the monitoring ports in close proximity in the forward and backward stages of the interferometers. (1) Installation of the monitoring ports increases the dimensions of the device; and (2) A device loss is increased because of the leakage of light to the monitoring ports.